1. Field of the Invention
The present invention relates to an air-intake device having an air-intake pipe connected to an air-intake passage formed in an engine body of an engine for leisure vehicles such as motorcycles, three-wheeled vehicles, all terrain vehicles, or personal watercraft (PWC), and an engine for a leisure vehicle equipped with the air-intake device.
2. Description of the Related Art
In one type of an engine mounted in leisure vehicles such as motorcycles, three-wheeled vehicles, all terrain vehicles, or personal watercraft (PWC), at least two-air-intake passages are arranged within a cylinder head, and an air-fuel mixture (fresh air containing a fuel) is guided from the air-intake passages to a combustion chamber through air-intake valves. More specifically, the engine is provided with at least two air-intake valves per cylinder and is configured such that the air-fuel mixture is supplied from the two air-intake passages arranged within the cylinder head to the combustion chamber through these air-intake valves. In such an engine, the air-fuel mixture is drawn to the respective air-intake passages of the cylinder head through an air-intake device having a common air-intake pipe within which a throttle valve is mounted (see Japanese Laid-Open Patent Application Publication No. 2000-204953).
Typically, a connecting portion of the air-intake passages which are connected to the air-intake pipe has a cross-section of substantially an elongated circle shape to allow the two air-intake passages to form a single air-intake passage, while the air-intake pipe has a cross-section of substantially a perfect circle shape in view of the relationship with a throttle valve openably (or pivotally) mounted within the air-intake pipe. The air-intake passage and the air-intake pipe, the cross-sectional shapes of which differ from each other, are connected to each other through a connecting member called a “holder” (or insulator), or the like. Specifically, one end portion of the conventional holder has a cross-section of substantially an elongated circle shape to conform to that of the air-intake passage, and an opposite end portion thereof has a cross-section of substantially a perfect circle shape conforming to that of the air-intake pipe. In addition, an intermediate portion of the holder has a cross-section which gradually changes its shape from the substantially elongated circle shape to the substantially perfect circle shape.
While the fresh air is flowing through the holder, the air flow is disturbed due to a fluctuation in a pressure loss in the passage of the intermediate portion in which the cross-section gradually changes its shape from the substantially elongated circle shape to the substantially perfect circle shape. As a result, air-intake efficiency decreases.